The combination of growing demand for wireless communication services, and increasingly scarce radio frequency spectrum, necessitates more efficient and flexible use of spectrum.
An example communication system comprising a satellite based access node (AN) and ground based user terminals (UT) and gateway terminals (GAT). One challenge in such scenarios is how to efficiently share the medium between the access node and terminals. In situations where centralised control is possible, the access node may coordinate access to the channel e.g. using some orthogonal access scheme, such as time division multiple access (TDMA) or frequency division multiple access (FDMA). Other methods include code division multiple access (CDMA), orthogonal frequency division multiple access (OFDMA), and single carrier frequency division multiple access (SC-FDMA). In the case of OFDMA and SC-FDMA the channel is divided across the band into subcarriers, and each user is allocated one or more subcarriers. Such schemes conventionally require very tight time and frequency synchronisation between all the transmitters in order to maintain orthogonality between the users' signals.
In many situations of interest, the communications medium may be dispersive in time or frequency (or both). One example of such a channel is the multipath mobile radio channel, where relative motion of transmitters and receivers can induce Doppler offsets. Reflections of radio signals from the environment also results in the superposition of many copies of the transmitted waveform, each with its own time delay, attenuation and phase offset. Another example of a channel which can introduce significant time and frequency offsets is in the context of communication with a satellite, e.g. in low earth orbit (LEO). In this situation, the time-of-flight to the satellite can vary significantly depending on the relative position of the satellite with respect to the transmitter. Furthermore, the high speed of the satellite (as seen from a fixed point on the ground) induces large Doppler offsets. In such situations, the channel can impose (potentially time-varying) time, frequency and phase offsets to each of the transmitted signals. In some situations the frequency offset may be time-varying, according to some frequency rate (measured in radians/second2 or Hz/second).
The Doppler induced frequency offset between the access node and a terminal is dependent upon geometry and the relative motion between the two entities. Consider a communication system with channel Doppler and time offsets induced through the system dynamics. In this example all terminals are transmitting modulated signals within the same designated frequency allocation at the same time. These may be either single or multiple carrier transmissions. The access node is travelling towards a first terminal T1, thus inducing a positive Doppler shift on the uplink signal and moving it to a higher frequency at the receiver. The access node is travelling away from a second terminal T2, thus inducing a negative Doppler shift on the uplink signal and moving it to a lower frequency at the receiver. The access node is directly above terminal T3 and there is negligible Doppler shift. All signals require time to travel from terminal transmitter to access node receiver, introducing a delay proportional to the link distance. Doppler induced shifts may be significant in relation to available uplink bandwidth and spacing between subcarriers. For example, for the case of a 400 MHz carrier frequency, a terminal directly ahead of a LEO satellite based access node is shifted upwards in frequency by approximately 10 kHz. In conventional systems these effects can reduce performance by introducing inter-carrier interference. The performance degradation increases with induced frequency shift.
The channel induced offsets may not be known a-priori to the transmitters, which means that attempts to transmit using orthogonal access schemes can be defeated by the channel. Although the transmitted signals are orthogonal, they may be non-orthogonal when they arrive at the receiver. This causes multiple-access interference, whereby the signals from different transmitters mutually interfere at the receiver. This is can seriously degrade the performance of the system.
One approach to address this problem is to estimate the relevant time, frequency and phase offsets at the access node based receiver and use a feedback channel to provide these estimates to the corresponding transmitters so that they can pre-compensate for these effects. However, this approach must operate fast enough such that the channel parameters do not change substantially from when they were estimated to when they are used. Moreover, this approach may also increase receive processing complexity at the access node. Increased access node complexity is highly undesirable for some applications, such as those where the access node is located on-board a satellite.
Another approach is to use fixed guard bands and guard intervals to provide sufficient time and frequency separation so that no matter what offsets are introduced by the channel, the signals from different transmitters do not interfere. This approach is simple, but can present a problem if a spectrally efficient system is required. For example, in the case of a LEO satellite system operating at 400 MHz, the maximum induced Doppler shift will be approximately ±10 kHz, thus requiring 20 kHz of guard bandwidth.
In the case of a satellite deployment, a large access node field of view offers the potential to communicate with a wide geographical distribution of terminals. This can lead to a diverse range of channel conditions between the access node and each terminal. In many instances during a satellite pass the use of fixed guard bands will be overly conservative, e.g. when the terminal is directly beneath the satellite and the induced Doppler shift is negligible. Moreover, for systems that aim to compensate for channel affects, the diverse range of channel conditions creates a need for dynamic and accurate channel estimation methods.
There is thus a need to provide methods, systems and components for improving the spectral efficiency of multiuser multicarrier communications networks, or at least to provider a useful alternative to current systems.